This invention relates to a branching unit and system for underwater communication in which optical signals can be selectively routed between branches.
Branching units for underwater optical communication systems normally have three branches and are normally arranged to through-couple one fibre pair connected to one branch to a fibre pair on another branch, which fibre pairs form a main highway for optical data signals. In order to route specific signals to a spur terminal connected to another branch such signals are propagated with a specific wavelength and can be diverted to the other branch by means of a wavelength division multiplex system (WDM) which also permits signals to be sent from the spur terminal and to be injected at the specific wavelength into the main highway. This system is effective but does not permit maximum usage of the optical capacity of the main highway if no traffic is required to or from the spur terminal and does not permit selective access of the spur terminal to the main highway. The present invention has been arrived at from a consideration of these deficiencies and seeks to provide a branching unit which enables selective routing of optical signals between branches of a branching unit and also an optical transmission system employing such branching units.
In EP-A-635950 there is disclosed a branching unit in which various switching modes can be achieved by activation or inactivation of erbium doped fibre amplifiers (EDFA's) permanently installed in each fibre and which when activated overcome the effect of an optical attenuator in series with it in that fibre. The various EDFA's are powered from the sources which are powering the various optical cables, through bridge-type rectifier circuits.